This invention relates to a coupled cavity type traveling wave tube, and especially an attenuator to be provided in slow-wave circuits thereof.
A coupled cavity type traveling wave tube serves as a source of high power in microwaves and millimeter waves frequency band and thus is widely used as an output tube in the earth station for satellite communication.
To minimize possible intermodulation, most of the traveling wave tubes currently employed in circuits of large-capacity and high-performance communication such as satellite communication are operated in a region of high linearity, that is, in a small signal operation range where the output power level of the tube is low, so that the small signal gain versus frequency characteristic is an important factor for performance of the tube. However, the coupled cavity type traveling wave tube has a disadvantage that its small signal gain vs. frequency characteristic involves considerable degree of ripple as compared with that of a helix type traveling wave tube. Such ripple appearing in the small signal gain vs. frequency characteristic is caused by a reflected wave produced in the slow-wave circuit of the tube. A coupled cavity type slow-wave circuit develops a reflected wave more readily than a helix slow-wave circuit because of considerable variation in the characteristic impedance at the points where it is connected to an input waveguide and to an output waveguide as well as at the points where main cavities are coupled to matching cavities. A larger reflected wave causes not only greater gain ripple but also oscillation at a frequency where the positive feedback loop gain is above a value of 1. A coupled cavity type traveling wave tube according to the prior art technique has achieved stable operation with the aid of what is called a sever attenuator which severs the slow-wave circuit into sections of an input slow-wave circuit, intermediate slow-wave circuit and an output slow-wave circuit for complete isolation of propagation of circuit waves from one section to another. The purpose of using of this attenuator is to divide the slow-wave circuit into sections for suppressing the loop gain of each section down to a level smaller than 1 as well as for minimizing possible oscillations and gain ripple.
Severing the slow-wave circuit into a plurality of sections is very effective for achieving stable operation of the tube so long as the purpose is prevention of oscillation, but on the other hand, it has caused other troublesome effects in that reflected waves produced in the severed sections adversely affect the tube in such a manner that the resulting gain ripple reaches a noticeable level.